1. Field of the Invention
The present invention is concerned with noncovalently functionalizing carbon nanotubes using strong acids and functionalized polyaromatic molecules in order to increase their solubilities and/or conductivities.
2. Description of the Prior Art
Carbon nanotubes (CNTs) have shown great promise for conductive trace applications, especially in printed electronics. Printed CNT traces offer a number of benefits over traditional metal traces, including ease of application and of mechanical flexibility. However, “raw” CNTs are usually produced in a very disordered and impure powder, and must be purified and dispersed to create the conductive (or semiconductive) “inks” used to print the CNT traces. Getting CNTs to remain dispersed in solution, however, can be a challenge. CNTs very strongly attract each other due to van der Waals forces, causing them to agglomerate and fall out of solution. In order to create useful CNT inks for printing, processes must be developed to ensure that the CNTs remain dispersed.
Several methods have been used to make carbon nanotubes (CNTs) more dispersible, including oxidation processes, the use of surfactants, covalent functionalization with solubilizing groups, and non-covalent functionalization. Of these methods, non-covalent functionalization has the least effect on the electronic properties of the carbon nanotubes. Covalent functionalization creates defects in the pi network of the CNTs, which adversely affects their conductivity. Similarly, oxidation of the nanotubes will negatively affect the electronic characteristics of the CNTs, as the oxidation damages the tubes and could decrease their size. The addition of additives to the solution, such as surfactants, can also disrupt the electronic properties of the final ink-printed CNT films. To reduce this effect, post-applications treatments, including repeated washings, of the printed CNT films are necessary, which creates extra steps, yield lost, and large amounts of waste, and still may not restore the conductivity of the original CNTs.
Several methods of non-covalently functionalizing carbon nanotubes have been published. If the functional group is a liquid, then simply stirring at a raised temperature can be effective. Some solids can be melted with the carbon nanotubes but many solids decompose before melting, which is particularly the case for many polyaromatic hydrocarbons. Sonication can also be used to temporarily disperse CNTs in a solvent. Sonication can be used in conjunction with another functionalizing method, since it temporarily breaks up the carbon nanotube bundles and allows the functionalizing groups to get between the CNTs. However, strong or prolonged sonication has a tendency to damage carbon nanotubes, which likely results in less than desirable electronic properties.
There is a need for improved methods of solubilizing carbon nanotubes while preserving, and even enhancing, their conductivity.